Leading U.S. senators expressed alarm on Thursday at the scale and speed of China's land reclamation in the South China Sea and said a formal U.S. strategy was needed to slow or stop the work.

In a letter to U.S. Secretary of State John Kerry and U.S. Defense Secretary Ash Carter, Republican Senators John McCain and Bob Corker and Democrats Jack Reed and Bob Menendez said that without a comprehensive strategy "long-standing interests of the United States, as well as our allies and partners, stand at considerable risk."

They said China's land reclamation and construction in the South China Sea's Spratly archipelago gave it the potential to expand its military reach and was "a direct challenge, not only to the interests of the United States and the region, but to the entire international community."

The letter said Gaven Reef had grown about 28 acres (114,000 square meters) in the past year and previously submerged Johnson Reef was now a 25-acre (100,000-square-meter) "island." Fiery Cross reef increased in size more than 11-fold since August.

"While other states have built on existing land masses, China is changing the size, structure and physical attributes of land features themselves," the letter said. "This is a qualitative change that appears designed to alter the status quo in the South China Sea."

It said any attempt by China to militarize the artificial islands could have "serious consequences" and could embolden Beijing to declare a new air defense zone in the South China Sea like it announced in 2013 in an area contested with Japan.

The senators, who head the Senate Armed Services Committee and the Senate Foreign Relations Committee, said the strategy should lay out "specific actions the United States can take to slow down or stop China's reclamation activities..."

China claims about 90 percent of the potentially energy-rich South China Sea. It has defended its reclamation work there, saying it is not seeking to overturn the international order. The Philippines, Vietnam, Malaysia, Brunei and Taiwan also have overlapping claims.

Chinese reclamation work is well advanced on six Spratly reefs and workers are building ports and fuel storage depots and possibly two airstrips. Experts say this will not overturn U.S. regional military superiority but could allow Beijing to project power deep into the maritime heart of Southeast Asia.